Time to Burn?

Prescribed fires increasingly are being used to reduce hazardous fuels, a major objective of the National Fire Plan. However, despite advancing technology and ever-improving models, determining the best time for a prescribed burn is still a challenge.
     After gathering data and applying models, the fire manager may pick up a bit of duff, or organic material, squeeze it firmly and check the moisture before giving approval to begin the burn. The fire manager uses past experience, weather information and a "feel" for the current conditions as a final check.
     The effect of fire on the forest floor can vary from merely removing the litter to totally consuming the duff, which exposes the mineral soil and alters the surface soil structure. Fire managers often design prescribed fires to leave a portion of residual duff material to protect the mineral soil. Duff thickness and moisture content are the most important factors in determining duff consumption during fires. Using a measured duff moisture content in models like the First Order Fire Effects Model from the USDA Forest Service, fire managers can improve the estimate of duff remaining after a burn.
     Now there is an additional tool available. The DMM600 duff moisture meter provides reliable, real-time measurements of duff moisture content.

HOW IT WORKS In comparison to other woody fuels, duff moisture content has greater spatial and temporal variation. Small precipitation events and heavy dew accumulations that have negligible effect on the moisture content of large fuel can significantly increase the moisture content of fine fuels and litter. Subsurface duff can lose moisture through evaporation much more quickly than the large woody fuels. Due to subtle differences in canopy closure, slope, aspect and micro-topography, duff moisture levels can vary significantly across the landscape, even at the hill slope scale. These variations make it important to use real-time duff moisture measurements to estimate duff consumption and, more importantly, desired post-fire duff depths.

     Basic field use. The sample is collected from the portion of the duff layer just above the soil mineral horizon and is pushed through a No. 4 mesh sieve that fits in the opening of the sample chamber. Passing the duff through the sieve breaks up large organic fragments and removes sticks and rocks to allow for more uniform packing. Once the sieved material fills the chamber, the cap is put on and the compression knob turned until an audible indicator signals the sample is properly compressed. Upon proper compression, the instrument automatically takes the measurement, which is immediately visible in the display at the base of the instrument. Readings are displayed in real-time only; measurements are not stored. Total time needed to sieve and measure each sample is about 30 seconds.
     Operation principles. The tough, lightweight DMM600 is a portable, battery-powered sensor that was developed from frequency domain reflectometry technology, commonly known as FDR. When proper pressure triggers a measurement, a high-frequency signal of 42MHZ is applied to the wave guide at the base of the sample chamber, and the sensor electronics detect the change in frequency of the reflected signal. The frequency change depends on the dielectric constant of the medium adjacent to the wave guide. Since the dielectric constant of the medium varies with moisture content, the frequency change can be easily related, through a simple calibration function, to provide a moisture content measurement. The unit's microprocessor uses a factory-supplied calibration to convert the frequency to a volumetric moisture content and displays the value in the LCD readout.
     Air voids in the organic material may reduce the apparent dielectric constant and/or create a poor contact between the duff and the waveguide located at the base of the sample chamber. Using the meter's compression feature on sieved duff ensures each sample is pressed evenly against the wave guide, which reduces measurement variability. An audible signal tells the user when correct compression, 15 pounds, is applied and the measurement is complete.

DMM600 meter accessories CD-ROM containing video instruction, PCDMM software, instruction manual and Excel spreadsheet for calibrations Softsided, padded carrying case with screwdriver and spare 9-volt battery 6-foot, 9-pin serial cable

Specifications Size: 3-1/2 inch diameter 10 inch length Weight: 3.7 pounds Sieve: No. 4 mesh, .203 inch 3-inch diameter Battery: 9-volt alkaline Battery life: Over 2,000 uses Accuracy: ±5% for full-scale range Resolution: 1% volumetric moisture content

     Calibration. The factory-supplied calibration for the DMM600 is derived from laboratory measurements of the volumetric moisture content of duff from eight different forested sites. Given the elevation differences of these sites, the cover species varied to include spruce (Picea engelmannii), alpine fir (Abies lasiocarpa), lodgepole pine (Pinus contorta), larch (Larix occidentalis), ponderosa pine (Pinus ponderosa) and Douglas-fir (Pseudotsuga menziesii).
      Because the individual calibration curves were similar, the data were combined to develop a single, standard calibration curve (see below.) The error bars indicate that the measurement accuracy decreases as moisture content increases. The accuracy is approximately ± 4% at 60% volumetric moisture content and approximately ±1.5% at 30% volumetric moisture content. It's recommended that the average of samples from several nearby locations be used to reduce the effects of natural variability.

The meter's response to changing moisture content is best described with a quadratic calibration equation      Volumetric moisture content = 5.288 + 5.905 (frequency) - 0.142 (frequency2) where frequency is the DMM600 readout in megahertz. User-derived calibrations can be determined using the laboratory procedures described in the DMM600 instruction manual and the DMM600 calibration Excel spreadsheet provided in the PCDMM software package. User-defined calibrations are entered into the PCDMM interface and loaded to the DMM600 through a serial port connection. Studies done on eastern hardwood duff from Massachusetts show little deviation from the standard calibration curve. It is likely that the standard calibration curve will meet the needs of most fire managers across a range of vegetation types.

DMM600 standard calibration curve The variability is smallest in the lower moisture/higher frequency area of the curve, where accuracy is most critical.

     Volumetric versus gravimetric. The basic operation of the DMM600 gives the volumetric moisture content of the sampled duff. Using a simple calibration process, the gravimetric, dry weight-based moisture content — the measurement most commonly used by fire managers — can be added to the instrument readout.
     A value for duff bulk density is needed for this calibration process. Fire managers may choose to use a known bulk density value or determine one from local conditions, using the equation bulk density = dry weight/volume. Each coefficient in the standard calibration equation is divided by the bulk density.
     The three gravimetric coefficients are entered into the PCDMM software and uploaded to the DMM600 via a serial port connection on the base. Because duff bulk density is relatively constant, this calibration process can be completed prior to going to the field to make duff moisture measurements. When each field measurement is made, the standard volumetric moisture content and the user-defined gravimetric moisture content are alternately displayed in the readout.
     Duff moisture content is critical information for fire managers making operational and planning decisions for prescribed burns. The DMM600 duff moisture meter provides dependable duff moisture content data in the field and for input in predictive modeling programs.

The portable electronic duff moisture meter provides duff moisture data in the field.

Peter R. Robichaud, Ph.D., P.E., is a research engineer at the USDA Forest Service Rocky Mountain Research Station in Moscow, Idaho. He can be reached at the Forestry Sciences Laboratory, Rocky Mountain Research Station, 1221 S. Main St., Moscow, Idaho 83843; 208-883-2349; e-mail: probichaud@fs.fed.us.

Jim Bilskie, Ph.D., is the soil physicist at Campbell Scientific Inc., Logan, Utah.

The use of trade or firm names in this publication is for reader information and does not imply endorsement by the U.S. Department of Agriculture of any product or service.

This manuscript was written and prepared by a U.S. government employee on official time and therefore is in the public domain and not subject to copyright.

Originally published as:

Robichaud, P.R.; Bilskie, J. 2003. Time To Burn? Wildfire, May/June 2003. International Association of Wildland Fire.

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USDA Forest Service Rocky Mountain Research Station Moscow Forestry Sciences Laboratory 1221 South Main Street, Moscow, ID 83843 http://forest.moscowfsl.wsu.edu/